This circuit detects the darkness and turns on the buzzer accordingly.

1. Dark Detector circuit
Project by:
Manje Gowda K.R
Vivek J
Lavanya M.S
Madhurishree
Maharaja Institute of Technology-Mysore

2. Topics
 Components used
 What is LDR
 Circuit Diagram
Working
 Applications

3. Circuit Components
• +5 to +10 volts supply voltage
• 555 IC
• 1M,100K,1K ohm Resistors
• Light Dependent Resistor (LDR)
• 1000pF Capacitor
• Buzzer

4. Light Dependent Resistor (LDR)
An LDR is a component
that has a (variable)
resistance that changes
with the light intensity
that falls upon it. This
allows them to be used in
light sensing circuits.

5. Circuit Diagram

6. Working
When the light falls on the LDR, its resistance will be
very low. So the current
from the power supply does not choose R1=100K ohm
resistor path.
But when the LDR detects the Darkness its resistance
increases. So the current
from the power supply chooses R1=100K ohm resistor path.
 When the terminal 4 receives the supply voltage, 555 IC
will be triggered
making the buzzer to produce an alarm sound.

7. •Automatic street light control
•Automatic intensity control of street
lights
•Automatic lights control system in
industry
•Protection against no light for security
system based projects and many others.
Applications

9. Any Questions?